We have utilized loxP/Cre strategy to specifically delete exon II of NMHC II-B in a tissue-specific manner. A mouse line, B-loxP/loxP, in which exon II of NMHC II-B was flanked by loxP sites, was generated. NesCre, a transgenic mouse line harboring the Cre recombinase expression cassette under control of the nestin promoter, was crossed to B-loxP/loxP mice to obtain B-loxP/loxP NesCre, which allowed removal of NMHC II-B in the brain, but not the heart. The B-loxP/loxP NesCre mice are smaller than wild-type littermates, develop severe hydrocephalus, and demonstrate abnormal neuronal cell migration without showing any cardiac defects. B-loxP/loxP mice were also crossed to a transgenic mouse line, alpha-MHC-Cre (MHCCre), in which the Cre expression cassette is driven by the alpha-cardiac myosin heavy chain promoter, to generate B-loxP/loxP MHCCre mice, in which NMHC II-B was specifically ablated in the heart. B-loxP/loxP MHCCre mice are presently being characterized, but have demonstrated cardiac ventricular septal defects and no brain abnormalities to date. We are also interested in understanding the role of Pitx2a in the regulation of cell cycle progression in HeLa cells. Binding of high-risk human papillomavirus (HPV) E6 protein to E6-associated protein (E6AP), a cellular ubiquitin-protein ligase, enables E6AP to ubiquitinate p53, leading to p53 degradation in cervical cancer cells such as HeLa cells. Pitx2a, a bicoid-type homeodomain transcription factor, can bind to HPV E6 protein and inhibit E6/E6AP-mediated p53 degradation. Deletion of the Pitx2a homeodomain abrogates its ability to bind to HPV E6 protein and to induce p53 accumulation in HeLa cells, suggesting that the homeodomain of Pitx2a is essential for inhibition of E6/E6AP-mediated p53 degradation. Recombinant Pitx2a can block E6/E6AP-mediated p53 degradation in vitro, indicating that this function of Pitx2a is independent of its transcription activity. Pitx2a does not regulate Hdm2-mediated p53 degradation, since Pitx2a does not affect p53 protein levels in HPV-negative cells such as HCT116, U2OS and C33A cells. In addition, Pitx2a-induced p53 is transcriptionally active and maintains its specific DNA binding activity in HeLa cells. Taken together, these findings suggest that, by binding to E6, Pitx2a interferes with E6/E6AP-mediated p53 degradation, leading to the accumulation of functional p53 protein in HeLa cells.